Most likely, there’s a preamp tube that going or gone. At least, that’s the easy problem to solve.

It’s also possible that an interstage coupling capacitor is bad. When those go, they will cause problems like you describe.

It could also be an issue with a solder joint, although I very rarely see that with Mesa gear.

You should have a tube layout chart (if not, download the manual from Mesa). I would swap preamp tubes with the other clean channel. If the problem moves, it’s the tube. If it doesn’t, it’s something else in the channel.

Good luck.

Greetings from Austin, Tx

Ken